Q: Once we created the workflow for a transaction including emails, if I need to customize the workflow/activities and the emails for this particular transaction, how do I do that? (Just changing the email verbiage or adding additional or deleting an activity.)
A: Changing verbiage on an email isn't a problem. You can change that at any time - the activities use the assigned template at the time it goes out so having it setup for "Send Manually (with Preview)" will allow you edit it before it goes out for that particular property.
Adding/Deleting activities is the bigger challenge. When you start a workflow it makes a "copy" of the current workflow process and goes through it. If you add items after the fact or delete items then those have to be done separately.